1. Field of the Invention
The present invention relates to a clock frequency control method and device which selectively outputs one of various clock signals with different frequencies to access an IC card of a certain kind such that the selected clock frequency is suited to the IC card of the kind. Moreover, the present invention relates to an IC card reading/writing apparatus using the clock frequency control device.
2. Description of the Related Art
Conventionally, there are two methods of changing one of a plurality of clock frequencies to another while preventing the occurrence of a spike-like signal. That is, one method is to change a frequency-dividing factor, which is used to convert the same clock frequency into a controlled clock frequency, so that the controlled clock frequency is output (which method will be referred to as the first method). As disclosed in Japanese Laid-Open Patent Application No. 5-83101, the other method is that one of two non-synchronized clock signals is changed to the other or vice versa, and the resulting clock signal is output (which method will be referred to as the second method).
However, according to the standards of IC (integrated circuit) cards, various kinds of IC cards which work with different clock frequencies are permitted. In addition, a variety of clock frequencies for IC cards are defined by the IC card standards. Among such IC card standards is the Japanese Industrial Standards (JIS) X 6304: 1999. In order to achieve accessing the IC card of every kind, it is necessary that an IC card reading/writing apparatus be provided with the function to change one of a plurality of clock frequencies, specified for such IC cards, to another.
When producing an operating clock signal specified for an IC card by changing the frequency-dividing factor for a certain oscillation frequency according to the first method, the first method has the problem in that it is difficult to set the frequency-dividing factor at a predetermined value, and the specified clock frequency cannot be obtained.
Moreover, according to the IC card standards, an IC card reading/writing apparatus must satisfy the requirement that, when changing the original clock to a new clock, the creation of a clock signal with its pulse width, which is less than the pulse width of the original clock, should be inhibited.
The second method, which is disclosed in Japanese Laid-Open Patent Application No. 5-83101, provides the function to switch between the two non-synchronized clock signals with different frequencies. However, there is no teaching in Japanese Laid-Open Patent Application No. 5-83101 concerning the configuration of clock frequency control hardware that changes one of three or more clock signals with different frequencies to another and outputs the resulting clock signal, in order to enable the reading and writing of IC cards of various kinds.
An object of the present invention is to provide an improved clock frequency control method and device in which the above-described problems are eliminated.
Another object of the present invention is to provide a clock frequency control method which is capable of selectively outputting one of three or more clock signals with different frequencies without changing considerably the pulse width of the output clock signal before and after the clock frequency change.
Another object of the present invention is to provide a clock frequency control device which is capable of selectively outputting one of three or more clock signals with different frequencies without changing considerably the pulse width of the output clock signal before and after the clock frequency change.
Another object of the present invention is to provide an IC card reading/writing apparatus which includes a clock frequency control device configured to selectively output one of three or more clock signals with different frequencies without changing considerably the pulse width of the output clock signal before and after the clock frequency change.
The above-mentioned objects of the present invention are achieved by a clock frequency control method which comprises: the steps of providing a plurality of clock output selectors which respectively provide output clock signals based on input clock signals with different frequencies at respective selector inputs; and controlling one of the plurality of clock output selectors in response to an operating clock request to provide a selected clock signal among the output clock signals in accordance with a first indication of the request, wherein the controlling step includes: deactivating, when the first indication of the request is changed to a second indication, the selected clock signal in synchronism with a corresponding one of the input clock signals; and activating, after the selected clock signal is deactivated, a second selected clock signal in response to the second indication of the request in synchronism with a corresponding one of the input clock signals.
The above-mentioned objects of the present invention are achieved by a clock frequency control device comprising: a plurality of clock output selectors which respectively provide output clock signals based on input clock signals with different frequencies at respective selector inputs; and a controller which controls one of the plurality of clock output selectors in response to an operating clock request to provide a selected clock signal among the output clock signals in accordance with a first indication of the request, wherein the controller deactivates, when the first indication of the request is changed to a second indication, the selected clock signal in synchronism with a corresponding one of the input clock signals, and activates, after the selected clock signal is deactivated, a second selected clock signal in response to the second indication of the request in synchronism with a corresponding one of the input clock signals.
The above-mentioned objects of the present invention are achieved by an IC card reading/writing apparatus which includes a clock frequency control device for providing a computer system with read/write functions to read information from an IC card and to write information to the IC card, the clock frequency control device comprising: a plurality of clock output selectors which respectively provide output clock signals based on input clock signals with different frequencies at respective selector inputs; and a controller which controls one of the plurality of clock output selectors in response to an operating clock request to provide a selected clock signal among the output clock signals in accordance with a first indication of the request, wherein the controller deactivates, when the first indication of the request is changed to a second indication, the selected clock signal in synchronism with a corresponding one of the input clock signals, and activates, after the selected clock signal is deactivated, a second selected clock signal in response to the second indication of the request in synchronism with a corresponding one of the input clock signals.
The clock frequency control method and device of the present invention are effective in selectively outputting one of three or more clock signals with different frequencies in response to the operating clock request according to the kind of the inserted IC card, without changing considerably the pulse width of the output clock signal before and after the clock frequency change.